Aircraft control and indicating apparatus



Nov. 8, 1955 o, H, scHUCK ET A1. 2,723,089

AIRCRAFT CONTROL AND INDICATING APPARATUS Filed Deo. 18. 194'? Gttorneg United States Patent O AIRCRAFT CONTROL AND INDICATING APPARATUS Oscar HugofSchuck and Ross C'. Alderson, Minneapolis, Minn., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application December 18, 1947,. Serial No. 792,560' 13 Claims. (Cl. 22M- 77) This invention is concerned with apparatus that is provided in an aircraft which: apparatus automatically stabilizes the aircraft withy .respect to an axis, thereof by operation of a control surface.. There4 exists apparatus for stabilizing an aircraft with respect to an axis thereof that operates in such. manner .thatv the 'existence of a steady-state condition which tends to change the attitude of the aircraft about said axis isv countered` by a steady-state displacement of a. control surface 'of said aircraft. Where such control surface is steadily displaced, a continuing or steady force must be applied by the apparatus to resist the tendency yof Athe force on the displaced control surface, dueto the air stream, to return it to a position of zero aerodynamic force. vUnder these conditions, wherey the control surface must be steadily displaced, the aircraft is :said to be in an out of t-rim condition.

An object of our invention 'concerns improved means for providing a compensating etfect when steady-state out of trim conditions, of said aircraft exist.. By-y the means which we provide, the: stabilizing apparatus is relieved ofy the, burden of supplying a fsteady force. or moment which is necessary to resist the force 'or moment of the air stream on the control surface.

A control surface of an aircraft isdn many cases provided with an auxiliary control lsurface termed a trim tab. A further object of our invention isto provide for the positioningof a trim tab` by .improved control means, dependent upon the .presence of. an. out of trim condition in said' aircraft, whereby thel required steady displacing moment on the control surface. isudevel'operl byv the moment produced by" the trim tab. The-stabilizing. apparatus is therebyl relieved of 'the task of counteracting the steady force or momenttending to streamline the displaced control surface.

A further object of `our inventionisv to. provide automatic vmeans for positioning; the control surface trim tab, dependent upon the steady-state control surface moment, acting in such a wayI asv to reduce the steadystate moment to zero.

A further object of your 'invention is to` provide improved means for positioning a trim `tab of a control surface of an aircraft upon. the existence `of a steadystate reactive .moment of the air 'stream on"v the control surface and to indicate when the reactive moment'- lis adequately counteractedt byv the opposing rrromentv developed by the trim tab as positioned.

A further object ofy our invention .is to provide 'an indication in accordance' with the. magnitude and direction of the force or moment due to the out of triml 'condition.

A further object of our invention is to provide an indication in accordance with. the magnitude and` direction of the reactive moment developed by the air -stream on a control surface of an. aircraft, Wl-iic'hl reactive momentv is being counteracted by the control surface operatngV means, and manually` operable means. for posibtioningv a trim tab associated with` the. control surface 2 so that-the position of the tab may lcounterbal'ance said reactive moment thereby to reduce said indication. I These andi other objects of -the invention will be perceived upon consideration ofthe accompanying description and drawing of a preferred 'embodiment of our invention.

The sole figure `corn'prisi-n'g the `drawings is a diagram of `rour invention applied to an aircraft having an elevator for controlling the position of the aircraft about the pitch axis.

Referring to the ligure, the aircraft' is controlled about the pitch axis by a main control surface or elevator 1'0. The elevator is operated by cables 11 extending from the cable drum 12. In they ligure th'e cables are discontinuously illustrated by a break in the cable. The cable drum 12 is operatively driven by an elevator servo motor 14 which in turn is controlled by the elevator servo motor amplifier' 15. The amplifier and servo:- motor arrangement is such that the direction of rotation of motor 14 depends on the voltage applied to amplifier 1-5 fand? may be similar to that disclosed in Patent 2,425,734 datedy August 19, 1947. Amplifier 15 is provided with: signal input terminals connected to leads 16 and '117 and power input terminals connectedI to leads 18 and 19 which are further connected to a source of voltage such as the inverter (not shown) of the aircraft through a supply line 100. The direction of rotation 'of the servomo'tor 14 depends upon` the phase relationship between the voltage l'across leads 16 and 17 and that across leads 118 and 19.

The signal input circuit of amplifier 15 extends from lead 1f6, pitch deviation-servo balance network 21, lead 17', tor amplifier 1'5. The pitch deviation-:servo balance network 21'c'omprises a pitch deviation potentiometer -221 aridaser'vo balance potentiometer 27. Potentiometer .22 has a resistor 23 which is connected across the ends of lthe secondary winding 25 ofv a transformer having-a primary winding 40; The potentiometer 27 has a '-re'- sister zslconnected acrossl the ends of secondary winding 251 Sin parallel with. resistor 23. Potentiometer 22v is provided with a contacter 24 which is adjustable over the surface 'of resistor '23. Potentiometer 27 Ais. provided withv a contacter 29 Awhich isv adjustable over the re- .'sist'or 28. Ift `may be seen that the network 21 is in the form of a Wheatstone bridge with the wipers 24 and29 representing the output member for the bridge. The Wiper 24 is connected through lead 16 to ampli- .fier I5. Wiper `29v is connected by means of lead 17 tov amplifier 15. The wiper 24 is adjusted by a vertical gyray 30'.

The vertical gyro 30 may be ofthe 'conventional three degrees of freedom type in which the rotor 31 is'mounted for rotation about an axis which is perpendicular to the earth. The rotor 31 is carried by 'an inner gimbal ring 32; The inner gimbal ring 32 is trunnionedy in an outer gimbalring 33 about anaxis that is at right angles to the axis of the Vrotor 31. The outer gimbal ring in turn has trunnions 34 and 351 which are supported by bearings carriedv by the craft. The axis of' trunnion 34, 35 is perpendicular to the axis of rotation of the inner gimbal ring32.

The wiper .24 is secured to the trunnion 34 and operated thereby upon movement ofthe aircraft `about its: pitch axis. The wiper 29 of the servo' balance potentiometer 27y is driven through a follow up connection from servomo'tor 14y by any suitable means such as intermeshing gears 36, 377y carriedy by the servomotor shaft 38 andthe operating shaft 39-for wiper 29.

auxiliary control surface or trim tab 42 is pivotally carried near the trailing edge of the elevator 10. The trim tab 42. is voperatively connected to cables 43 which vfor a. simplified illustration are shown to extend from fa sheave 44. Adjacent 'tor the. sheave 44 andy relatively fixed thereto is an additional sheave 46. The sheaves 44 and 46 are rotatably carried by the pivotal axis of the elevator 10. Extending from the sheave 46 is an additional operating cable 47 which engages a single pulley 48 carried by shaft 49. While the arrangement shown is sufficient for understanding this invention, in practice the operating means for the trim tab 42 includes provisions, such as a differential, between the elevator actuator and the trim tab actuator so that the position of the trim tab relative to the elevator will not be affected by movement of the elevator. The pulley 4S and its shaft 49 are operatively driven by a trim tab operating motor 52 through a reduction gearing 50 and an overriding clutch or friction clutch 51 which is intermediate the pulley 48 and reduction gear 50. The clutch includes a driving member 51 and a driven member 60. Associated with the driven member is a manually operable controller 61 which may be operated to adiust trim tab 42 and to disengage the driven member from the driving member. Such overriding clutch may comprise a driving member, a driven member, with rollers between said members which on initial movement of the drive member are displaced to effect the engaged relationship of the members. The driven member is provided with means to disengage the two members and to allow manual rotation of the driven member. Such clutch may be of the type disclosed in the patent to Kastner, 1,028,360.

ri`he trim tab operating motor 52 may be and is illustrated as a capacitor type induction motor having a squirrel cage rotor 53 which coacts with two phase windings 54, 55. The winding 54 is connected in series with a capacitor 57 across the line 100 and the winding 55 is connected to the output terminals of an amplifier 56. The amplifier 56 is provided with signal input terminals 66 and 67 and power input terminals 64 and 65 the latter two being connected to the line 100. The direction of rotation of the trim tab motor 52, as well known, depends upon the phase relationship between the voltage across signal input terminals 66 and 67 of amplifier 56 and that across line-excited winding 54 of motor 52.

The signal input circuit to amplifier 56 extends from terminal 67, lead 70, elevator hinge-moment sensing network 72, lead 80, to terminal 66 of amplifier 56. The hinge-moment sensing network 72 includes a resistor 73 which is suitably connected across the ends of a secondary Winding 75 of a transformer having a primary winding 76 which is connected to the line 100.

Resistor 73 is carried in insulated relation by the elevator cable drum i2. A contacter M is operative over the surface of resistor 73 and is carried by a hub 78 mounted on the servomotor shaft 353. interposed between the hub 78 and the drum 12 is a resilient member or spring 79 which is secured at one end to the drum 12 and at the other end to the hub 78. The spring 79 serves to bias the wiper 74 toward the electrical center of resistor 73 and further serves as a torque transmitting means between the hub 7S and the drum 12. Lead 70 extends from terminal 67 to the wiper 74. The secondary winding 75 has a center tap 77 which is connected by means of lead 89 to terminal 66 of amplifier 56.

Operated in parallel to the amplifier 56 which controls the trim tab motor 52 is a hinge-moment indicating arrangement. This hinge-moment indicating arrangement comprises a D. C. meter S1 operated from a modulation bridge 82. The D. C. meter 81 may be of any suitable type whose pointer may be deflected to one side or the other from a zero position depending upon the polarity of the signal applied to its input terminals. The meter 81 is connected across the output terminals of the modulation bridge 82. The modulation bridge may be of the type disclosed on page 567 of AC Bridge Methods by B. Hague, 1943 edition. Such bridge has signal input whose phase depends upon the phase across terminals 83 and 84 of bridge 82. The signal input terminals 83 and 84 of the modulation bridge S2 are connected by means of leads 87 and 88 to the wiper 74 and center tap 77 of the hinge-moment sensing network 72.

Operation Operation of the apparatus will be considered with respect to a normal position or condition. When the aircraft is ying in a level position, wiper 24 of pitch deviation potentiometer 22 and wiper 29 of servo balance potentiometer 27 are at the electrical centers of their terminals 83 and 84 and terminals S5, 86, the latter two l.

being connected to the line 100. By means of the bridge the two A. C. inputs are converted into a D. C. output wise direction.

respective resistors 23, 28. At this time both wipers are at the same potential and there is no signal applied to the input terminals of amplifier 15. Similarly, wiper 74 of hinge-moment sensing network 72 is at the electrical center of resistor 73 and is consequently at the same potential as center tap 77 of secondary winding 75. Consequently, there is no input signal applied to amplifier 56. The elevator control surface 10 and its trim tab 42 are in normal or horizontal position.

lf the aircraft is tilted about the pitch axis due to a transient condition the vertical gyro moves wiper 24 from its center position to apply a signal to amplifier 15 and servomotor 14 which operate wiper 29 to balance the amplifier input circuit and position the elevator. As the elevator restores the craft toward normal, the gyro moves wiper 24 toward center thereby causing an unbalance in the amplifier input circuit to cause reverse movement of the servomotor which moves wiper 29 to rebalance the input circuit and the elevator to normal position.

In the following operation to be described, on the other hand, it is assumed that a steady-state out of trim condition exists, This steady-state out of trim condition may result from the change in the position of the center of gravity of the aircraft as a whole. For example, if part of the weight carried by the craft is moved toward the after part of the cabin the front or nose of the aircraft will tilt upwardly. As the craft tilts upwardly the wiper 24 is stabilized by the vertical gyro 30 and maintains its position in space consequently the wiper 24 will apparently move toward the right end of resistor 23. The wiper 24 will now be at a different potential than wiper 29. Assuming the voltage between wiper 24 and wiper 29 to l be in phase at this time with the voltage between leads 18 and 19 supplied from the line 100 the amplifier 15 will operate. Operation of amplifier 15 at this time will cause the elevator servomotor 14 to rotate in a clockwise direction whereby the elevator control surface 10 is lowered or moved in a clockwise direction. Servomotor 14 also through the follow up connection positions wiper 29 towards the right until network 21 is in balanced condition at which time the amplifier ceases to operate.

Under the applied down elevator the aircraft tends to move from its upwardly inclined direction toward the horizontal. This movement of the aircraft toward the horizontal causes the vertical gyro 30 to apparently move wiper 24 toward the left from its right position. At this time wiper 24 is again at a different potential from wiper 29. Since the wiper 24 is toward the left from wiper 29 the signal between wipers 24 and 29 is considered out of phase from the original signal derived from network 21. In response to this out of phase signal, amplifier 15 causes servomotor 14 to drive cable drum 12 in a counter clock- Servomotor 14 also operates through the follow up connection to position wiper 29 toward the left until the network is again balanced. It is evident therefore that as the contactor 24 is moved toward the center from its rightward position that the amount of down elevator is decreased. It is further evident that elevator 10 cannot assume its normal position, otherwise the craft would immediately nose upward due to the shift on the center of gravity. Consequently, a point is reached where the contactor 24 assumes a position to the right from its r normal position. Follow up wiper 29 is also positioned toward the right from .its normal position so that `the amplitierinputis balanced.

Since the elevator ,10.is in a vslightlydown position there is a .constant force bythe .air stream tending ltolrnovethe elevator 10 back toits normal position. I nthe ,type of servomotor vdisclosed `in the aforementioned patent, the servomotor shaftisheld in locked .position-when it is not operating; consequently, hub 78 will be locked against movement. Force ofthe air .stream on the elevator 10 is communicated through the operatingl cables 11 to the cable drum 12. A torque or moment is applied to the cable drum 12 tending to rotateit. The resilient connection or .spring 7:9 betweendrum 12 and hub 78 permits the force on the elevator 1 0 to relatively rotate cable drum 12 with respect .to hub 7 8. Resistor 73'carried by drum 12 will have its electricalcenter moved with respect to contactor 7 4 carriedfby hub 78. The potential of V"contaeter 7,4 will :not `be vthe same ,as center'tap 7 7, consequently a voltagesignal ;Will be appliedltolhe,inputterminals 66 and67 of the-trim tab motor .ampli-tier S6. The phase of the voltage .betweencontactor 74and center tap 77, .and .thereforel'between A'terminals ,6.6 and 67, ,with ,respect tothe voltage yacross. 1-noter winding 54is such asto cause the .trim .tab mot yr .5 2 .toprotate ,in .such adirection that its movement as applied through lthe ,reductiongearing 5,0, clutch 5.1 pulley 4.8, cables 47, .sheave io-, .sheave 44, land cables 45, will position the trim .tab .42.in .anppward. direction. M

The force of the air stream vonthe.trir r1.tab,42 tends yto rotate the elevator 10 in a clockwise direction whereasthe force of the air stream .on the main control surface 10 tends to rotate .the elevator in acounterclockwise rdirection. As the vclockwisemornent on .the trim tab 4 2 increases, it decreases vthetorque applied I tothe cable drum 12.r The resilient connection `7,9" therefore causes drum 12,to .rotate in a clockwise direc/tion .tending to V,bring .the center tap of resistor 73 in alignment `with:contactor 74.

A sllong as .thecenter tap fof resistorV 73 ,andcontactor 74 are out of alignment, k'z'ontactor 74 and .the'tcenter tap 77 are at a different potential, consequently ,the trim .tab motor amplifier 5,6 Ais operated. The 4trim tab motor 52 `is continuously operatcdgby 4the amplifier .56 until the trim tab surface 32 has assumed va :positionsuch thatthe moment. onits ,surface is., equal and. opposite tothe moment on the main control surface 1Q, at which time vthecenter tapof resistor 73.has been moved into alignment withcontactor 74. At this time there `isno diierenceofgpotential between contactor 74 and the center tap 77 andfnohinput signal is .provided tothe amplierSo. The-ampliiier 56 ceasesto. operatethe motor 5 2.

.the .indicadores .the dieter .ldooroasos- .The .meter ..81 will( be 'at 'the zero position when the center of resistor 73 is in alignment with contactor .7.4.2112 which time ,the ,required .steady-Stato .moment .of rthe control .surface is bgngtodtirely .developed vby the .aerodynamic action of ..fho trim-tab- Themetor S1 may therefore Serve as an .indication .as .to the .relative l.istarloo which the ttrim tab .m thepositioned untilthe moment required of the main .Qontrol surface .10 is entirely developed by the vtab4 2.

Thooporatioohas .beendesoribod whore .afstoady-stato outof trim 4Qpos' `o n rexists due to a .Changein position of lthe centertofgravity toward the tail of the aircraft. The apparatusoperates in an opposite manner -to provide up elevator .when the .conter .of gravitvfhas been -shifted to- .wardthefnose of the ailtxaft. Thismovementforwardof tho ootorof .gravity .is accompanied by an opposite 1111- balance of the network 21 which is accompanied also by a downward positioning of the trim `tab 42 whereby the ,mqmenton the ltr im ta b.4 2 tending to rotate thecontrol .Surface 1 0 ina counter clockwise direction balances the .moment on Ath, e.oo ntr 1 Surface 1.0 .tending to .rotate Ait in With the yn noment onlthe 'trim tabequalto that Lon .the

main elevator 10, but ofopposite direction, there .is no differential tension inthe operating cables 11 to theele- .vator 10, consequently, the center tap of resistor 73 will remain in alignment with contactor ,74. v

'I'he aircraft llies .with its ,main control elevator in a slightly depressed positionand with the .wipers 24 of-the deviation potentiometer and the .wiper 2 9 of the servo balance potentiometer to the right of the .no rrnal position. Since the forces on the Kupper and lower elevator operating Cables are 110W equalized, fhoro is no .forooon the drum 12 and the motor shaft 38 rtending vtowrotate the same, consequently, the servomotor 14 ismelieved from lthe load which had been applied to it `p revio'usto the positioning of the trim tab 42.

-Duringtho time that the hneormomont Sonsinanotwork 72 has boon drawings-:Signal tothotrim Ristoratopliior 56 it has also .Supplied ofSena1-foftho-.mo-d ation bridge 82. The D. C. meter 8 1 has-been o eratedfbythe output ,from the modulation bridge .toindicatethe ,magnitude and .direction ofthe force .of the' yair.stream .on the main control surface. As the ymoment applied ,by servo motor 14 to ,the main control surface te moved, .due .to thedCyclo,pmeritx of amonient .by the trim tab .42, Ithe vsignal from the 2network 7a2 ,decreases and ya. clockvvis-edirection. lfhe .indicating meter 81 will have ,moved from its `center position in an opposite direction t0 indicat e the ytn axirniim moment on the main control surfaceand will be restored to its 4center position as the moment developed by the trim tab becomes that required to develop the moment required to keep the main control s nrfacein the ,position necessary to maintain the .selected attitude.

lI t ,would appear ^that whenever kthe control surface 10 is displaced that there is a force on this surface which tends'to move Athe drum 1 2 with respect to the hub 78. Thus ,it would appearthat the trim Ytab motor 52 vis oper- ,atedto positiQn thetrimtab ft2 even though the apparatus is, fu n,cti oning to correct fora deviation in pitch due to a transient ycondition as distinguished from a .steady-state ,condition d ue to shiftingof the centerof gravity ofthe ,aircraftpr other variation in flight condition. It will be recalled, however, thatthe trim tab motor drives the trim ,tab 4 2 through areduction gear 5 0 so that thetrim tab 42 is positioned ata very slow rate. For short-time transienteiects such asthoseresulting from the tilting of the aircraft, due to a disturbance in the air, the trim tab 42 is movedonly asmall amount. Further, it is considered that thereare as many disturbances tending to move the nose of theaircraftupwardly as there are tending to move `it downwardly; consequently any small motions ofthe trim tab 42 ,inone .direction .would be equalized by small motionsin the otherdirection. The trim tab therefore :remains vsubstantially in its steady-state position since the transientconditions over a period of time balance out.

While the contactor 74 has been illustrated as controlling the motor through a potentiometer and amplifier, it,isevident thatit may coact .with spaced contacts which may replace the resistor 73 `to control the motor 52, yand that anyother device suitable for measuring the control surface hinge-moment and producing an electrical output dependent thereon may be used for controlling the trim tab actuating Vmotor yas described above. Likewise, the motor may be .of any type suitable for the service required. Furthermore, the inclusion of a tachometer generator, to .permit velocity feedback for greater control system stability, is an obvious extension of the procedures Vdescribed above.

it is now evident that we have provided a novel trim tab positioning and indicating apparatus in which the positioning of the trim tab relieves the servomotor that operates the main control surface from required load and the indicator indicates continuously the magnitude of the load.

While we have disclosed our invention in one form of apparatus, we do not wish to limit our invention to the details of the yembo d,1 l311,t illustrated. We therefore would limit our invention only by the statements in the appended claims.

We claim as our invention:

l. Control apparatus for an aircraft comprising: operating means; transmission means driven by said operating means adapted to move a main control surface and responsive to a force applied to said control surface; a motor adapted to operate an auxiliary control surface; electrically operated indicating means displaceable in either direction from a normal position; variable electric signal producing means having an output that may be varied in phase or polarity and magnitude and actuated by said transmission means; means operated by said signal means for controlling said motor; and further means operated by said signal means for controlling said indicating means whereby the reactive force from said control surface to said transmission means may be observed.

2. Control apparatus for an aircraft having a first adjustable means for controlling its position about an axis comprising: actuating means for said adjustable means; control means for said actuating means comprising a balanceable electrical network including a control member and a controlled member; attitude means responsive to the movement of the aircraft about said axis; operative connections from said attitude means to said control member; operative connections from said actuating means to said controlled member; further adjustable means for controlling the aircraft about said axis; means responsive to an effect caused by a steady force tending to turn the aircraft about said axis as the result of the operation of said first adjustable means; indicating means controlled by said force responsive means and operated in proportion to the force; and additional means controlled by said force responsive means for operating said further adjustable means to modify the effect on said force responsive means.

3. Control apparatus for an aircraft comprising: a main control surface electric motor means; an auxiliary control surface actuating means; transmission means driven by said motor means, and adapted to be connected to said control surface, said transmission means including two rotationally relatively displaceable members biased toward a normal position; a second motor means including a friction clutch operative upon relative displacement of said members for controlling said actuating means; and manually operable means for overriding said friction clutch and operating said actuating means without altering the position of said second motor means.

4. Control apparatus for an aircraft having a main control surface, and an auxiliary control surface said control apparatus comprising: a first main control surface motor means; a second auxiliary control surface motor means; a main control surface transmission means operated by said rst motor means; said transmission means including two relatively dispiaceable members biased toward a normal position but relatively displaced in proportion to the reactive force on said main surface; means operative upon said relative displacement for controlling said second motor means; and means including an indicator operative upon said relative displacement for providing an indication in proportion to the extent of said displacement whereby the reactive force on the transmission means may be observed.

5. Control apparatus for an aircraft having a main control surface and an auxiliary control surface said control apparatus comprising: a main control surface power means; an auxiliary control surface actuating means; a main power transmission means having means responsive to the reactive force applied to said transmission means from said main surface; operating means for said actuating means controlled by said responsive means, the operation of said auxiliary surface reducing the force on said transmission means; indicating means controlled by said responsive means whereby the force on said transmission means may be observed; and manually operable means for independently operating said actuating means.

6. Control apparatus for an aircraft having a main control surface and an auxiliary control surface said apparatus comprising: a first main control surface electric motor means, including a motion transmitting device having two relatively displaceable members; an alternating current pick-olf device including two elements one element carried by one member and the other element being carried by the other member for providing a signal variable in phase and magnitude in accordance with the direction and magnitude of the relative displacement; an auxiliary control surface slow acting electric motor means, said auxiliary surface serving to reduce said displacement; indicating means operable in accordance with the magnitude and direction of a signal; and means for controlling said second motor means and said indicating means from said pick-off device whereby the effect of said auxiliary surface may be observed on said indicator.

7. Control apparatus for an aircraft having a main control surface and a trim tab carried by said surface said apparatus comprising: a first electrical motor means for operating said main control surface; a second electrical motor means for operating said trim tab; a iirst electrical signal providing pick-off means having two relatively displaceable elements; means for relatively displacing said elements; means responsive to said signal from said pick-off means for controlling said first motor means; a second electrical pick-od device having two relatively displaceable elements; means responsive to the torque applied to said first motor means for displacing said elements; and means controlled by continued displacement of the elements of said second pick-olf for controlling said second motor means.

8. Control apparatus for an aircraft having a control surface with a trim tab, said apparatus comprising: a control surface electric motor means, said motor means including a transmission means having two biased connected relatively movable members; a variable impedance controlled by the relative movement of said members to provide a signal in proportion to the magnitude of the reactive force applied to said transmission means from said surface; an indicating means controlled by said variable impedance in proportion to the direction of and the magnitude of said force; and manually operable means for adjusting said trim tab, whereby the effect of reducing said force by positioning said tab may be observed on said indicator.

9. Control apparatus for an aircraft comprising: a first main control surface power means; a balanceable control means for said power means including a main controller and a follow up controller; attitude means responsive to change in its angular position about one axis thereof and adapted to operate said main controller; means driven by said power means for operating said follow up controller: av trim tab power means; a rst power transmission means including two relatively displaceable but mutually restrained connected members; a second balanceable control means for said second power means in which the first of said two members is driven by said rst power means and the second member is positioned with respect to said first member in accordance with the reactive force on said transmission means from said control surface, whereby said trim tab power means is positioned by said second control means.

10. Control apparatus for an aircraft comprising: a rotatable control surface power means; control means for said power means having an operation initiating element and a rebalancing follow-up element; operating connections from said power means to said follow-up element; attitude sensing means responsive to the movement of the aircraft about an axis and operatively connected to said initiating element; a rotatable control surface actuator coaxial with said power means; a resilient member between said power means and said actuator for transmitting turning effort from said power means to said actuator; an auxiliary control surface power means; and control means for effecting operation of said auxiliary power means and responsive to the deformation of said resilient member, said auxiliary power means operating an auxiliary surface supported by the control surface to reduce said deformation.

11. In combination in an aircraft having an elevator control surface provided with a trim tab, said elevator surface normally tending to assume a streamlined position: a motor for positioning said elevator; connecting means between said motor and said surface comprising two relatively movable members yieldably connected together, one of said members being connected to said motor and the other to said surface; means responsive to a condition for controlling said motor to position said surface; a controller positioned in accordance with the relative movement of said members when said motor tends to maintain said surface in other than said streamlined position; and means controlled by said controller for electing movement of said trim tab for causing the position which said surface tends to assume to be changed from said streamlined position.

12. Apparatus for positioning a trim tab of a main aircraft control surface, said apparatus comprising: a reversible motor for positioning said trim tab; a speed reducing device between said trim tab motor and said trim tab; main surface power means; motion transmitting means for positioning said main surface from ,said power means including a member having two elements, one connected to said power means and the other to said surface and resilient means interconnecting said two elements and biasing one element toward the other, whereby said two elements are relatively displaced depending on the force applied to said surface by said power means; and directional control means for said trim tab motor including means connected to said two elements and effective on the relative displacement thereof to cause said trim tab motor.

13. In an automatic pilot for an aircraft in flight having primary and secondary control surfaces subject to aerodynamic forces for controlling the attitude of the aircraft about one of its axes of control, said automatic pilot comprising: sensitive means for developing a signal voltage upon deviation of the craft from a desired flight condition; an electrically operated servomotor operable in response to said signal voltage to move the primary control surface to re-establish the desired ight condition; means for developing a displacement signal upon operation of said servomotor opposing said signal voltage; means responsive to both signals operating said servomotor for delivering the required torque to position and maintain in position said primary surface against aerodynamic force thereon; a second electrically operated servomotor positioning said secondary surface to receive an opposite aerodynamic force thereon to reduce said torque requirements; and an electrical circuit means affected by the torque requirements of said first servomotor during the development of a steady state signal by the sensitive means controlling the energization of said second servomotor to cause the latter to operate continuously as long as the torque requirements of said rst servomotor are abovea predetermined value.

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